Re: bug#24514: 24.5; Lispy backtraces

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: Clément Pit--Claudel <clement.pit@gmail.com>
> Date: Sun, 4 Dec 2016 14:27:59 -0500
> 
> >> The C implementation of backtrace-frame seems to be linear in the index of the requested frame, so a Lisp implementation of backtrace would be quadratic in the depth of the stack trace.  Would a new function backtrace-frames that returns all frames at once be acceptable?
> > 
> > But such a backtrace-frames function would have to be implemented in
> > C, right?  And you wanted to move the implementation of "backtrace" to
> > Lisp, AFAIU.  So it sounds like we will be replacing one C primitive
> > with another, or did I miss something?
> 
> I think you're correct. It would seem good to have the flexible primitive backtrace-frames available, and it must be in C; then we can move backtrace itself to lisp.
> 
> The idea is that enumerating frames must be done in C, but printing them doesn't need to be done there.

So would it perhaps make sense to rename 'backtrace' into something
like 'backtrace--internal', and make it accept one more argument, the
function to apply to each frame, which is now hard-coded as 'prin1'?
Would that allow you to implement 'backtrace' in Lisp and also
implement whatever application you had in mind, by calling
'backtrace--internal' passing it your own function instead of 'prin1'?